Method of and means for communication



Apil 23, 1940.

C. W. HANSELL METHOD OF AND MEANS FOR COMMUNICATION Filed sept. 1, 19563 Sheets-Sheet l INVENTOR C. w. HANsz-:LL BY .l M

ATTORNEY n. Qt Q NN dmv April 23,- 1940- -c. w. HANsELl. 2,198,248

METHOD 0F AND MEANS FOR COMMUNICATION Filed Sept. 1, 1936 3 Sheets-Sheet2 HIGH lll

ATTORN EY April 23, 1940.

c. w. HANsELL 2,198,248

METHOD OF AND MEANS FOR COMMUNICATION Filed Sept. l, 1936 3 Sheets-Sheet5 Ely. 4,

ATTORNEY Patented Apr. 23, 1940 UNITED STATES METHOD OF AND MEANS FORCOMMUNICATION Clarence W. Hansell, Port Jefferson, N. Y., assignor toRadio Corporation of America, a corporation of Delaware ApplicationSeptember 1, 1936, Serial No. 98,874

21 Claims.

This invention concerns a novel method of and means for signalling bymeans of wave energy, the amplitude, or phase, or frequency of which hasbeen modulated in accordance with signals and the resultant sidebandstransmitted with or without the carrier.

The novel method of and means for signalling, of the present invention,involves utilizing each sideband separately for signal reproductionpurposes to thereby obtain frequency diversity in the receiver .and allthe advantages to be gained thereby, such as reduction of fading and ofnoise. 'I'he absence of the carrier from the received energy reducesfading usually caused by carrier amplitude fluctuation and harmonicdistortion which is caused by beating of the carrier with the sidebandsin the circuits or during transmission. When the carrier is suppressedat the transmitter a new carrier may be introduced at the receiver andmay be regulated in amplitude so that best results are obtained when itis beat with the sidebands to render the signal.

Modulatcd wave energy from an ordinary amplitude or phase or frequencymodulated transmitter may be received, but the receiver of the presentinvention is also adaptable to the reception of modulated wave energyfrom a multiplex transmitter of any type, and in particular from themultiplex facsimile transmitter of the present application' and of myUnited States application #756,926 led December 11, 1934 of which thisapplication is a continuation in part. In the reception of any type oftransmission` in accordance with my novel method and means, the benefitsof frequency diversity afforded by frequency spacing of the sidebands,each of which alone is caused to reproduce the signal and both of whichaiect the indicating means jointly is obtained.

Likewise, the amplitude diversity effect of signals on two sidebands isobtained in the reception of any type of signals and this isparticularly of benefit when signals are multiplexed.

In my novel receiver, the sidebands are separated and separately beatwith the received carrier or with a new carrier introduced at thereceiver and separately demodulated to produce signal indications whichmay be combined. Since the two sidebands are demodulated separatelydistortions in the resultant signals cannot be caused by phasefluctuation in the sideband or carrier energy.

As stated above, my novel method of and means for demodulating waveenergy may be utilized with any type of transmitter but is particularlyadapted to a multiplex transmitter. 'Ihis transmitter comprises aplurality of brushes, preferably or twelve, which are arranged inasingle line to scan the message which is printed on a tape having anon-conducting backing. This tape has printed on its surface the messagecharacters in (Cl. Z50-6) control ten audio frequency currents, arethenL impressed on a common line circuit and passed through atransmitter for transmission over the ether, if a radio circuit is used,or over a line to a distant receiver if a land line circuit is used.

In order to reduce undesired fading due to the effect of carrieramplitude fluctuations, and to reduce harmonic distortions which causeinterference between channels, it is proposed to eliminate the carrierat the transmitter and to send out both sidebands, and at the receiverto re-introduce the carrier before detecting the signal waves. However,I also contemplate transmission of the carrier which is beat with thesidebands at the. receiver after said carrier is brought to a desiredconstant amplitude. f

In'receiving, the individual audio frequencies corresponding to eachchannel are separated by means of filters, and two rectified currentscorresponding to each channel combine to reproduce the message at ascanning head, which may, if desired, be identical with that used at thetransmitter. Thus both sidebands may be used without introducingdistortion due to phase fluctuations, and benefit is obtained from theamplitude diversity effect of using two sidebands on each channel.

'Ihe advantages of using such a multiplex facsimile printer arrangementas outlined above are as follows:

l. 'I'here is no need for synchronizing equipment to keep the terminaloflice transmitting and receiving equipment in synchronism, as must bedone in the full page facsimile system. It is only necessary to run thereceiving tape at approximately the same speed as the transmitting tape,and this may readily be judged by eye and set by hand.

2. The messages sent out by the multiplex facsimile system arerelatively secret, even without the provision of any special secrecyequipment. However, the messages may be made still more secret byproviding like cams at transmitter and receiver to continuously vary therelative positions of the ten transmitting and receiving points and byfrequently changing the assignments of the radio channels among the tenscanning points.

3. By using the multiplex principle, the keying speed on each channel isreduced to a small .iraction of that required to scan a given area in agiven time by the single channel method. If ten channels are employed,then the keying speed, or modulating frequency, ci each channel isreduced to of that required when a single channel is used. For thisreason, by he use of suitable equipment, the echo phenomena caused hymultipr. transmission in the radio circuit has less distor .ig effectupon the reproduction. In addition, the reduced speed of each channelperu mits the use of relatively simple transmitting and receivingequipment. At the transmitter, for eX- ample, there may be used the newphotocells manufactured by the Weston Electrical Instrument Companyunder the trade name Photronic, to operate relays and control audiochannels directly without requi mg any vacuum tubes or battery supply.At the receiver, ordinary electromagnetically operated vibrators willrecord as rapidly es will be needed for most circuits.

4.. By using a single line of type sent and received, with, let us papertape, very little, if any, scanning of blank. space is required.Therefore the total intelligence transmitted per unit frequency per unitof time is better than that obtained with the full. page facsimilesystem. Even on irly closely written messages the gain may easily beabout two to one, and, on the average, considerably more.

5. The facsimile method, as distinguished from code op rated printers,is relatively free from errors, and even when letters are mutilated suchfact will be recognized. Incorrect, unrecognizable errors andsubstitutions are practically absont. With the terminal equipment whichmay be used, the probability of whole letters being missed is greatlyreduced hy staggering the pickup recording points lengthwise of thetape. For a letter to be missed entirely, a fade or interference mustlast for the time required to pass several letters through theequipment.

G. The operation of the multiplex facsimile communication system doesnot require operators specially trained to be efficient in the Morse orContinental telegraph codes. It is only nec essary to employ skilledtypists to type messages on tape at the transmitter and to copy them onmessage blanks at the receiver. This should make possible a considerablereduction in central oice personnel cost.

'7. If it is assumed that the power of the transmitter, or its fullmodulation Swing, used on one channel is then if ten channels aremultiplexed on it the amplitude of the modulation allowable on eachchannel for ideal cenditions is 10%, and the power used per channel isonly 1%. In other werds. multiplexing theoretically reduces the powerper channel in pro portion to the square of the number of channels.Actually, with the multiplex facsimile scheme, considerable allowancemay be made for the diversity in phase of the audio modulating currentsand the timing of the marking intervals on the channels. That is, thesystem may be so set up that all the channels never mark, and all audiofrequency currents are never at their peak value with like polarity atthe same time, so that one might have a considerable increase in theamount of modulation per chan nel due to the iact that the modulationsdo not all add together in phase at the same time, and very seldom, ifever, would all channels be modulated at once. By using the equipmentwithout any particular regard to the phase relations oi the ten tones,the amplitude per channel can be set at onequarter the amplitude whichcould be allowed for a single tone. This corresponds to a pow per ofinstead of 1% of that allowable when all the modulation is concentratedon a single tone. Since in addition to time diversity in the channelsthere may be tolerated occasional slight over-modulation, the power perchannel may be increased to 10% without causing undue diiliculties.Since there are ten channels, the sum total of modulation therefore mayamount to 100% oi that obtainable on single channel. in other words, thepower eiiiciency of the mul tipler, system may be set up to beapproximately the same as the emciency ol a single modulated system.

8. The use of phase or frequency modulation rather than amplitudeiodulation enables obng five or six times the power obtainable withamplitude modulation with an equipment of a given construction cost.Consequently, the overall effectiveness oi the phase or frequencymodulated multiplex facsimile printer transmit er in accordance with thepresent invention is pproximately six times that of a single toneamplitude modulated transmitter. However, my novel transmission systemmay use amplitude modulation at the transmitter and demodulation at thereceiver as well as phase and frequency modulation.

9. At the receiver, use of automatic tuning control with the phase orfrequency modulated circuit permits the increase in selectivity ci thereceiving system to a point where there is admitted a frequency bandonly great enough to include the useful modulation.

The foregoing advantages will be more readily apparent from a reading ofthe following detailed description which is accompanied by drawingswherein:

Figures l. and 2 illustrate two diierent embodiments of transmittingcircuits for multiplex facsimile printer systems embodying theprinciples ci the present invention, the diierence in the two circuitsbeing mainly in the kind of tape used and the manner oi sending eutphase mod ulated Waves instead of amplitude modulated waves.

Figure 3 illustrates one form of receiving apparatus which can be usedto receive the phase or amplitude modulated waves.

Figure 4 illustrates simpliiied form of my receiver which is adapted tothe reception oi wave energy modulated in any manner.

In Figure lv there is shown a multiplex facu simile transmitter systemfor ten channels. A multi-tone generator I of any well known form isarranged to generate ten different audio ircquencies f1, fz, f3, etc.,one for each channel, which frequencies are individually passed bysending nlters to scanning contacts 3, which may be brushes or needlesarranged to bear down on a tape L'. Tape 4 is comprised on anonconductive ne tissue having written or printed thereon the message inconductive letters, this tape being passed underneath the scanningcontacts. Each channel is shown as having a separate contact 3associated with its filter and another Contact such as 3" which is in acom mon circuit with the other channels, whereby when the two contactsof each channel are connected together upon engaging a letter on thetape, an obvious circuit will be closed to send out the audio frequencycharacteristic of the Li i) channel over line 5, which line is common toall the channels.

Tape 4 may be madek of paper upon which is placed any suitableconductive solution, for example, potassium, 'ferricyanide and sodiumchloride. For that matter the surface may be rendered conductive bymarking with an ordinary lead pencil. The tape may consist of a thinfoil mounted upon a stout paper backing and', if so, the surface of thefoil should be covered with a non-conductive fine tissue and the messageplaced upon this tissue.. In such case a special solution of graphitemay be used for rendering the tape electrically conductive where markedby the stylus or pen. When the metal foil is used the foil may serve asa common contact for all the scanning points.

Instead of using paper or a metal foil, an endless metal tape may beemployed. This tape may be passed through a liquid solution, which, whendried, leaves an insulating film on the tape. Then, when writing ortypewritten matter is put on the film by marking with an electricallyconductive element such as graphite, in the same liquid as theinsulator, the insulating material will be pushed and washed aside andits place taken by conducting material. The tape may then be run underthe electrical contacts, its message sent, and subsequently it may bewashed, returned to the insulating solution, and the whole processrepeated.

It is preferred that the metal tape be of a color which is as differentas possible from the marking material so as to make the record readilyvisible to the operator. For economical reasons, however, a polishedsteel tape is desired. The solution for the insulating and markingmaterial should preferably be in water and slightly alkaline to preventcorrosion of the tape. The insulating material might be made of suchelements as water glass, sugar, alum, borax, cadmium sulphide, camphor,dextrin, starch, Egyptian blue, glue, mucilage, etc.

The marking fluid might be a colloidal solution of any conductingmaterial which is chemically stable in Water. For example, carbon,tungsten, silver and platinum might be used. If desired, the marking maybe done with a solution which reacts chemically with the material in oron the paper to deposit or precipitate out a conducting material. Ifdesired, solutions of deliquescent material, for example a solution ofdeliquescent salts which retain moisture and are conducting, might beused for passage under the transmitter contacts. In the latter casecoloring matter would be added to the solution in order to make itlegible on the transmitting tape. As an alternative to coloring the saltsolution, substances might be put in which would decompose due to thepassage of electrical current to form a visible record. The record wouldthen be developed by ,the current flowing through it as it passed underthe transmitting contact points. The record so formed would thusrepresent a true copy of the message as sent through the transmitter andso would be of value as a check on the functioning of the transmitter.This may be accomplished, for example, by using iron scanning electrodesand a marking fluid containing potassium iodide or a combination ofpotassium ferricyanide and sodium chloride.

The tape may be impressed with the message characters either by writingor by a typewriter whose keys hit upon a ribbon placed above the tapeand the thin tape then passed under the sending contacts. Ify desired,the tape may be wound around a drum 6, as shown in Figure 2, which drumis rotated to move the type past the scanning contacts. In using anarrangement such as shown in Figure 2, it is preferred to use metal tapeof the type mentioned above, whereupon itis only required that there beone scanning contact 3 associated with each channel, the other contact3" in common with all of the chan nels being in contact with the tapethrough the drum, whereby closure of both contacts will occur when 3engages a conductive symbol or letter on the tape.

As the tape passes beneath the sending contacts', there is produced aycomplex wave representing the sum of the waves of frequencies fi, fa,fz, etc., at any instant, dcpendinon upon th. number of channels closedthrough by the configuration of the lettery at that instant, whichcomplex wave is transferred over line 5 through a suitable audioamplification means such as l, and then through a land line 8 leadingfrom the transmitter to the receiver in the case of wire transmission,or to a radio transmitter in the case of radio transmission.

At the radio transmitting station there is a source of carrier frequency9 which preferably is controlled by a crystal I0. The carrier energy issupplied symmetrically to a pushpull modulator il, as shown in Figure 1of the drawings, in consequence of which the carrier is eliminated fromthe output I2, and only the two sidebands resulting from amplitudemodulation of the carrier and the original modulating frequencies areproduced, which are then amplified by power amplifier I4 and radiated bymeans of an antenna I5. When desired, a filter I3 may be applied as ahigh pass lter which cuts off at the carrier frequency so that themodulator frequencies and one of the sidebands are eliminated, the othersideband being amplified by the power amplifier I4.

The system of Figure 2 is very similar to that of Figure 1, except thatherein a steel tape is used instead of a paper or metal foil tape, andphase or frequency modulation is employed by means of suitable wellknown apparatus, herein conventionally indicated by box I 6. It ispreferred to employ the transmitter system of this figure instead ofthat of Figure 1, because of the greater amount of power which may beobtained over each channel.

For receiving signals sent out over the system of Figure 1, any suitablearrangement may be employed wherein a carrier Wave is reintroduced atthe receiver and the audio frequencies characteristic of the variouschannels filtered out to record the signals on a suitable scanning headwhich preferably is identical with that used on the transmitter. Onesuitable way of accomplishing this is disclosed in my United StatesPatent #1,751,584 to which attention is invited.

.Figure 3 illustrates the preferred form of receiver for receiving phasemodulated waves transmitted over the circuit of Figure 2. This apparatuscomprises a special phase modulation telephone receiver having two setsof audio frequency filters and two sets of rectiiiers or Thyratronsoperated from the output of the receiving filters. A description of thisreceiver and the operation thereof will now be given:

The incoming phase modulated radio frequency energy picked up by antennaI is increased in power in a high frequency tube amplifier 2, and theamplified energy is beat down by means of oscillations from 0 to anintermediate frequcncy in a detector of the electron discharge type Theintermediate frequency energy is further amplified in tube amplifier 4.The output of amplifier i is then divided into two portions, one ofthese portions being the carrier wave and the other being the sidefrequency waves produced by the multiplex modulation of carrier at thedistant transmitter. The new carrier wave is separated out from thecomplex signal by means of a neutralized piezo-electric crystal filter 5whose selectivity is so great that in the output there appears only theunmodu- 1'ated carrier the mean intermediate frequency. This carrier isthen amplified and limited in iter t so that its strength will remainconstant :ardless of variations in the strength of the carrier picked upby the antenna. The limiting to eliminate the eects of carrier fading.limiting, the carrier, whose strength is relatively large, is split intotwo compohaving equal strength but differing in phase by 0 or 180, oneof these components being applied cophasall.y to the control electrodesof the tubes of detector and the other component being appliedco-phasally to the control electrodes of the tubes of detector 8. Thestrength of the carrier introduced as described into the two detectorsis made relatively large.

carrier may also be supplied from a separate local oscillator O'connected by switch S to the lines L leading to detectors 'l and 8 inwhich case the lines are disconnected from G by switch S'.

Another portieri of the intermediate frequency energy is taken from theoutput of amplifier 4 and applied to high and low pass filters 9 and i3.These filters separate those frequencies lying above and below thecarrier frequency into two independent circuits. The carrier ofinteri'ate frequency may or may not be permitted to pass through thefilters but this s not important since if carrier energy does passthrough these filters it will reach the two detectors 'l and with astrength very much less than the strength of the carrier that is appliedfrom the lil iter i to the detectors. Assuming that there .3, er andlower side frequencies produced by the multiplex modulation and theseside freci s separately flowing in the two circuits, 1 e energy of onegroup of side frequencies is i p 'ed to the control grids of the tubesof both .I `s of detectors 'i and 8 in like phase relation. other set ofside frequencies is applied to ie control grids of the tubes of the twodetectors and fl reversed phase relation. Under these onditions, takinginto account the fact that detectors i and 8 are supplied with carrierenergy :Tn l or 180 phase relation, the outputs from the and 8 willdiffer in audio phase two detectors i 'cy O or i800. The outputs fromthe two detectors are passed through transformers Il and I2, each ofwhich has two secondary windings. One secondary winding of eachtransformer is connected in series with a similar winding on the othersformer with like polarity, and the other ondary winding of eachtransformer is conin series with the corresponding winding Aie othertransformer with reversed polarity.

Under thc-se conditions, if the conductive confecions from the low passfilter l0 are interrupted, it can be seen that the energy from high passfilter 9 will produce outputs from both detectors and 8 which will addtogether in one cf the serially connected output transformer circuitsbut oppose each other in the other serially connected outputtransformer. If, on the other hand, the energy from high pass filter 9is in stead interrupted but that from filter l0 allowed to pass thenthere will be obtained output energy in the secondary circuits of theoutput transformer in the other of the two serially connected outputcircuits. In other words, in each of the two output circuits energy isobtained corresponding to only one of the side bands produced by themodulation. When the mcdu lation consists of intermittent audio tones ofdifferent frequency produced by the scanning of the tape at thetransmitter one should normally find a corresponding audio modulation inboth of the serially connected output circuits. However, if radioconditions are such that one side frequency or sido band has faded outor been reduced in amplitude the corresponding side frequency orfrequencies on the other side of the carrier frequency may still bepresent. Only one side frequency corresponding to any one modulatedfrequency needs to be present in the signal picked up by the receiver inorder that there may be a corresponding output in one or the other ofthe serially connected output circuits. If desired, the series of keyedtone frequencies in each output circuit may be taken, separated from oneanother by means of band pass filters and then the energy of each toneconverted into a direct current. A similar treatment may be given to theseveral tones in the other output circuit. Under fading conditions thetone outputs in either of the output circuits may fail fairly frequentlybut only occasionally will the outputs from both output circuits failsimultaneously. In other words, there is a diversity effeet againstfading in using the two sidebands independently. If it is attempted todetect the modulation with the ordinary detector scheme wherein both theside frequencies are used to produce each audio output, very frequentfailures will occur due to the phase relation of the carrier wavevarying with respect to the sideband waves during transmission over theradio circuit.

In the description of the operation of Figure 3, so far given, it hasbeen emphasized that each side band produces an output in one of theserially connected output circuits and the other side band produces anoutput in the other of the serially connected output circuits. It mayalso be interesting to note the manner in which the two detectors I and8 of Figure 3 cooperate to bring about this result.

If at any particular time we have equal upper and lower side band inputswhich produce outputs from both of the two detectors then these outputsfrom the two detectors are made up of modulation frequency currentswhich are 90o different in phase. The 90 phase relation comes aboutautomatically due to the reversed polarity of input to the two detectorsfrom the low pass nlter. It is caused by the fact that reversal ofpolarity of one side band causes a change of one quarter cycle, at eachmodulation frequency, in the time required for a pair of conjugate sidefrequencies to reach any specified phase relation. In other words, if weare receiving a carrier wave modulated by a single audio frequency then,if we could reverse the polarity of side frequency current on one sideof the carrier, the effect would be to advance or reta-rd the time atwhich the two side frequency currents add (or oppose) by a time intervalequal to a quarter cycle of the modulating frequency.

Since the outputs of detectors 1 and 8, when both have an output, arealways 90 different in phase, due to reversed polarity of one of theside band inputs, it follows that the output in each of the seriallyconnected output circuits is made up of two output components ofcurrent, for each modulating frequency, and that these components mustbe added at 90, vectorially, in order to obtain the resultant output ineach circuit. Since the serial output connection is reversed, in theoutput of one detector in one of the serially connected output circuits,it follows that we must reverse one of our 90 vectors before we make thevector addition to find the resultant current or potential in thiscircuit. It is this reversal which gives us addition of the componentsdue to only one side band in each of the serially connected outputcircuits. 'I'herefore, if we could interrupt first one and then theother of the inputs from the high and low pass filters to the detectorswe would interrupt first one and then the other of the outputs to thetwo serially connected output circuits. That is, in each of the outputcircuits the components of output due to only one side frequency areadded and the components due to the other side frequency are opposed.

If we consider now the effect of the phase relation of the carriersupplied to the detectors, with respect to the modulation vector, orvector sum of side frequency components, we will nd that there is adifference of 90 in the relations of carrier and modulation vector inthe inputs to the two detectors. In other words, when the carrier andside frequencies cooperate to produce maximum amplitude modulation ofthe carrier in the input to one detector they produce maximum phasemodulation in the input to the other detector. Also, if the phaserelation of the carrier, with respect to the side frequencies, could bevaried continuously first one and then the other detector would beprovided with maximum amplitude modulation. The -amounts of amplitudemodulation in the inputs to the two detectors would vary up and downsinusoidally, and differentially, if the phase of the carrier were movedat a constant rate.

In long distance short wave radio communication the phase relation ofthe received carrier, with respect to the received modulation component,does vary in a more or less random fashion so that a wave transmittedwith amplitude modulation may arrive at the receiver with eitheramplitude or phase modulation, or both, in varying degree. The receivingsystem of Figure 3 gives a useful output when the received wave iseither amplitude or phase modulated because one or both detectors alwaysprovide an output regardless of the carrier phase.

We may think of detector 1 asan amplitude modulation detector and ofdetector 8 as a phase modulation detector, or vice versa, depending onspecific circuits and adjustments. One detector may then detectamplitude modulation and the other phase modulation. Then, by adding thetwo detector outputs, keeping in mind their constant 90 difference inphase brought about by reversing one side band input to one detector, wemay obtain an output from either of the two serially connected outputcircuits which is constant in value regardless of variation in thecarrier phase which turns amplitude modulation into phase modulation,and vice versa, in varying amounts as observed in practice on longdistance short wave circuits.

It would be fortunate if we could combine the outputs from the twoserially connected output circuits to obtain a single modulationfrequency output such as We would like to have for long distance shortwave telephone communication. Unfortunately this can not be done becausethe relative phases of the outputs in the two serially connected outputcircuits vary in accordance with variations of phase between theconjugate side frequency components which carry the modulation. i Attimes, due to random iiuctuations in relative phase or timing of the twoside frequencies, corresponding to each modulating frequency, the twooutputs would add and at times they would oppose. However, by keepingthe two outputs separate and using only their energy components,independent of phase, we may cause them to cooperate in providing aconmlon useful signal for purposes such as telegraph or facsimilecommunication. We may rectify the two alternating current outputs in thetwo serially connected output circuits to obtain direct currents whichcan be added or we can use each current to control a relay device suchas the Thyratrons in Figure 3, which cooperate in producing a usefulrecord or other indications of a transmitted message. t

We may note that the vector sum of the two detector outputs in either ofthe two serially connected output circuits remains unchanged even if thephase relation of the carrier, with respect to the side frequencies, iscontinually changing. For this reason there is no need for the usualfixed relation in phase between carrier and side frequencies. Largecarrier phase variations, during radio transmission, do not produce theusual modulation fading. Also, if desired, the carrier may be suppressedat the transmitter and its place taken by a. non-synchronous carriergenerated and introduced at the receiver.

The non-synchronous carrier must, of course, have a frequency close tothe exact frequency which a synchronous carrier would have. Otherwisethe beat frequency outputs between the carrier and each of the two sidefrequencies will not have the same frequency. If the carrier frequencychanges relative to the side frequencies there will be a differentialvariation in beats between the vcarrier and the side frequencies. If thevariations in the beat frequencies are very great then filters I3 and I4of Figure 3, for eX- ample, may not be wide enough to pass the signaloutput energies on their changed frequencies. In practice, it is notvery difficult to provide constancy venough to hold the output beatfrequencies within the filter bands. On the other hand, it is virtuallyimpossible to introduce a locally generated carrier into a doublesideband signal and maintain correct phase relations for producingconstant modulation of the carrier by the side frequencies. Thefrequency tolerance permissible in this case 1s zero and has never beenattained. For these reasons the possibility of using a non-synchronouscarrier at the receiver in the present rsystem makes double sidebandcarrier eliminated communication possible and so provides a greatlyimproved system. It is no longer necessary to provide a transmittercapable of the power output of both carrier and side frequencies inorder to obtain useful double sideband transmission.

Instead of converting each tone frequency from each output circuit todirect current in the ordinary manner, it is preferred to use theseenergies to control a pulsating direct current more suitable foractuating the vibrator of a multiplex facsimile recorder. Assuming, forexample, that one of the multiplexed channels employs a keyed tonefrequency of 1000 cycles, then under normal conditions 1000 cycle energywill appear in both of the receiver output circuits. If band passfilters I3, i4 will pass 1000 cycles, then keyed 1000 cycle energy willreach the grids of two 'Ihyratrons cr grid controlled glow dischargetubes l5, I6. The presence of audio excitation on the grids of theThyratrons will cause the gas within the tubes to be ionized when anodevoltage is applied. This will permit anode current to pass. If theanodes of the two Thyratrons are excited differentially by means of 60cycle alternating current from the transformer il, as shown, then solong as a 1000 cycle tone is present in both output circuits both tubeswill carry current on the positive half of the 60 cycle voltage appliedto their anodes. As a result, a pulsating direct current which ismodulated at the rate of "120 cycles will be applied to the coil of therecorder vibrator I8 and will cause the vibrator to record marks on asuitable recording tape corresponding to the markings on the transmittertape at the sending station. If either side frequency in the incomingsignal fades out, the recorder vibrator will still be operated if theopposite corresponding side frequency is still present though therecording will be perhaps slightly less efficient. A pair of audiofrequency band pass filters corresponding to lters I3, I4 will be usedfor each tone frequency used in the multiplex facsimile transmission. Ofcourse, if desired, the tubes l5, i6 may be of the high vacuum type orthey may be replaced by oxide rectifiers or any other suitable means forcontrolling a direct or a pulsating current, which can be used forrecording purposes. Preferably ten or twelve transmitting channels andten or twelve recorder vibrators would be used. The recorder vibratorsmay print the message on a paper tape by tapping a typewriter ribbon ora strip of carbon paper, or by any other means by which electricalenergy may be used to make markings on the paper.

It will be noted that the output of the special receiver noted abovegives two sidebands which are composed of the ten transmitted tones in asingle complex wave. The audio filters I3, i4 are really wave analyzers,there being one pair for each of the two tones, so that each of thecompleX sidebands are resolved into ten component tones, therefore,since there is an upper and a lower complex sideband, there will be anupper and a lower tone sideband for each transmitted tone. Figure 3shows only one of ten units coinposed of audio filters I3, I4,Thyratrons I5, I6, and recorder vibrator I6. As explained above, eachaudio lter passes one tone sideband. One tone sideband is impressed onthe grid of one Thyratron and the other tone sideband is impressed onthe grid of the other Thyratron. Thus, either sideband will cause a halfwave rectied current to flow through the recorder vibrator winding inpulses 60 per second. Each pulse causes the vibrator to move against alight spring, the spring lifting the vibrator between impulses. The tenvibrators are arranged so as to account for that element of the signalmark corresponding to the tone which was produced by scanning a singleelement at the transmitter. The vibrator engages a carbon paper tape ortypewriter ribbon, which rests in contact with a white paper tape, toreproduce the signal mark scanned.

In order that the receiver may be made more efficient, lt is preferredto apply an automatic tuning means for correcting the receiveradjustments and holding optimum conditions for reception. 1n doing thisa phase detector i9 may be used for detecting the phase relation betweenradio frequency energies of the carrier before and after the crystallter 5. Several different methods are well known in the art for varyingthe value of a direct current by means of the phase relation between twoalternating current energies. Any one of these schemes may be used inthe detector I. A direct current output is taken from detector IS whichvaries in strength or polarity or both in accordance with the phaserelation of radio frequency energies across input and output of thecrystal filter. The direct current energy is then used in any one ofseveral well known schemes for Varying the frequency of the firstbeating oscillator in the receiver. When connections and adjustments aremade correctly, any change in the frequency of the intermediatefrequency energy will be accompanied by a phase shift in the radiofrequency energy on the two sides of the crystal lter and this phaseshift will automatically react upon the first eating oscillator toreduce the change in intermediate frequency. Thus, once the receiver isadjusted to the transmitter, it will keep itself correctly adjustedwithout manual manipulation.

When the usual modulated wave, such as used in telephony is used, thetwo sidebands each produce in the output of l' and 3 signal currentswhich may be used separately or jointly in the recording apparatus shownor other indicating means such as phones.

Figure 4 illustrates a simplified forni of receiver accomplishingsubstantially the same results as that shown Figure 3. It will benoticed that the receiver in Figure i differs from that shown in Figure3 principally in the arrangement of the detecting circuits. Adescription of this receiver and its operation is as follows:

The incoming modulated radio frequency energy picked up by antenna I isincreased in power in a high frequency tube amplifier 2, and theamplified energy is converted by modulation with oscillations fromoscillator O in first detector 3 to a suitable intermediate frequency.The intermediate frequency energy is further amplified in tube amplifier4. The output of amplifier is then divided into two portions, one ofthese portions being the carrier wave and the other being the sidefrequency waves produced by the modulation of the carrier at the distanttransmitter. The new carrier wave is separated from the com piex signalby means of a neutralized piezo electric crystal filter 5 whoseselectivity is so great that in the output there appears only theunmodulated carrier cf the mean intermediate ircquency. This carrier isthen amplified and liniited in limiter ES so that its strength willremain constant regardless of variations in the strength of the carrierpicked up by the antenna. The limiting tends to eliminate the effects ofcar'ier fading. After limiting the carrier energy is further amplied andapplied through separate coupling means to detectors l and 8. 1t will benoted that in this receiver it is not required that the carrier have anyparticular phase relation l'or the proper operation of the detectors asoppose to the specific phase requirements of the receiver illustrated inFigure 3.

Another portion of the intermediate frequency energy is taken from theoutput of amplier 4 and applied to high and low pass filters 9 and I 0.These filters separate those frequencies lying above and below thecarrier frequency into two respective independent circuits. The carrieror intermediate frequency may or may not be permitted to pass throughthe filters but this is not important since if carrier energy does passthrough these filters it will reach the two detectors 1 and 8 with astrength very much less than the strength of the carrier that is appliedfrom the limitor 6 to the detectors. It is now Observed that in each ofthe detectors I and 8 are present only one sideband and the carrierfrequency and thus for detection purposes we have the equivalent ofsingle sideband reception wherein it is unnecessary to maintain aspecific co-relative phase relation between carrier and side frequenciesin order to obtain proper detection action. It will be further observedthat the system will function equally satisfactorily for eitheramplitude or phase modulation as the segregation of the sidebands intoseparate detectors converts either type of modulation into equivalentsingle sideband modulation.

The outputs of detectors 'I and 8 are separately coupled through theirrespective output circuits Il and I2 to suitable recording devices, inthe circuit illustrated, to a group of selective frequency multiplexrecorder vibrators, the operation of which was described with referenceto Figure 3.

One advantage obtained in this receiver is that it is substantiallyinsensitive to co-relative phase relations of carrier and sidebands.Thus there is no discriminatjon in output resulting from differentialphase changes of carrier and side frequencies caused by fading ormultipath transmission phenomena. Further, it is equally responsive toeither phase or amplitude modulation for the same reason.

Another advantage obtained in this receiver as in the receiver of Figure3 is the advantage of frequency diversity in that the receiver willdeliver a satisfactory output so long as the carrier and one of thesidebands is present and it is well known to the art that fadingphenomena over long distance transmission path discriminate betweenfrequencies differing by only a fraction of a percent in such a mannerthat it is seldom that both side frequencies of a particular modulationwill be simultaneously low as the result of fading but rather at aparticular instant due to the nature of the fading phenomena it islikely that one side frequency may be enhanced and the other reduced.Thus, the total output of this type of receiver may be eX- peeted toremain reasonably cons-tant in the presence of severe fading conditionsWhereas in a common type of receiver wherein carrier and sidebands areapplied together` to one detector, co-relative phase distortionresulting from fading will cause a wide variation in signal strength andobjectionable harmonic distortions in the output although the respectivefrequencies may be present in substantially proper strength.

It will be noted that the phase detector i9, controlling the frequencyof the heterodyning oscillator O is not essential to the properoperation of this receiver. However, it will be of considerableadvantage in maintaining alignment of the carrier and sidebands in theirrespective filter circuits as it is recognized that it is somewhatdiflicult to maintain alignment of the car- -rier in the piezo-electricfilter if an unstabilized heterodyning oscillator is used, particularlyin the reception of high frequency signals.

I claim:

1. A system of radio communication including a signal modulated carrierwave transmitter in which the carrier wave is suppressed but bothsidebands of the signal modulated carrier are transmitted, a receiverincluding means for beating each sideband independently with a locallygenerated carrier wave to produce two independent outputs whose strengthis substantially independent of the exact phase or frequency of thelocally generated carrier and means responsive to the two outputscooperatively to reproduce intelligible signals carried by themodulation.

2. A system of radio communication including a transmitter in which acarrier wave is modulated in phase by signal potentials and the carrierwave is suppressed but both sidebands of the signal modulated carrierare transmitted, a phase modulation receiver including means forproducing local oscillations of controlled amplitude and for beatingeach sideband independently with the locally produced oscillations toproduce two components of different phase whose strength issubstantially independent of the exact phase or frequency of the locallygenerated carrier and circuits responsive to the two components toreproduce intelligible signalscarried by the modulation.

3. A system of radio communication including a transmitter having meansfor modulating the length of a carrier Wave in accordance with signalsto produce at least two side frequencies corresponding to each signalfrequency, means for transmitting the resultant sideband frequencies, areceiver including a source of oscillations of a frequency of the orderof the frequency of the modulated carrier and meansI for separatelybeating each sideband with oscillations from said source of oscillationswhereby each sideband is detected independently of beats between thelocal oscillations and the other' sideband to produce two independentoutputs, each substantially reproducing the original modulationregardless of fluctuations in length of the received carrier, and meansconnected with said beating means for recording the sum of the twooutputs irrespective of their phase relation to render intelligiblesignals.

4. The method of reducing the effects of fading in radio communicationwhich includes the steps of modulating a carrier wave at the transmitterto produce two side frequencies corresponding to each modulationfrequency while suppressing the carrier wave energy, transmitting theside frequencies, receiving the side frequencies, producing localosciliations of large amplitude as cornpared to the amplitude of theside frequencies, beating the local oscillations of large amplitude witheach side frequency to separately dernodulate said side frequencies andcombining the resultant energy to reproduce the signals.

5. A system of radio communication including a transmitter in which thecarrier wave is suppressed and both sidebands of thesignal modulatedcarrier transmitted, a receiver including means for producingoscillations the amplitude of which may be regulated for beating withthe said sidebands, means for beating each of said sidebandsindependently with said produced amplitude regulated oscillations andmeans for utilizing the two energies produced by said beating action toproduce the signal.

6. A system of radio communication including a phase modulationtransmitter in which the carrie wave is suppressed and both sidebands ofthe sig al modulated carrier transmitted, a receiver including aseparate phase modulate wave fic-modulator for each sideband, a commonmeans for producing oscillations the amplitude or" which may beregulated for beating with the respective sidebands, means in eachdemodulatcr ior beating each of said sidebands independently with saidproduced amplitude regulated oscillations and means coupled with theoutput ci' each demodulator for utilizing the energies produced by saidbeating action to reproduce the signal.

'7. The n' n"hed oi reducing the of lading in radio communication whichincludes the steps of modulating carrier wave at the transmitter toproduce at least two side frequencies corresponding to each modulatingfrequency, transmitting said side frequencies, receiving said sidefrequencies, separately 1seating said side frecuencias with oscillationsof controlled amplitude to produce two detected outputs, controlling thefrequency of said oscillations in a sense to compensate ior phase orfrequency va "iatlons of the received wave, and utilizing said outputscooperatively to reproduce intelligible signals.

8. Ln a system for receiving Wave energy modulated in phase andcomprising carrier and sideband components, means for separating saidcarrier and sidehand components each from the other and passing theseparated components over separate paths, apair of detectors, eachdetector comprising two discharge devices each having a controlelectrode and an output electrode, means ior coupling one of the pathsin which sideband energy news to the control electrodes in the devicesof one of said pairs oi detectors, means coupling the path in which theother sideband energy 'lows to 'the control electrodes in the devices of'une other of said pair of detectors, means coupling the path in whichcarrier energy flows to corresponding electrodes in each or" the devicesl: `th detectors and an output circuit coupled to the cutout electrodein each of the devices of both or s d :airs of detectors.

9. lli a ase modulation receiver, a plurality ci pairs ol" electrondischarge devices each having a control a cathode and an anode, a sourceor" phase modulated oscillatory energy comprising carrier sidebands,means for applying one of said sidebands in phase opposition to thecontrol grids of the tubes oi each of said pairs of uces, means i'orapplying the other of said sidebands in phase opposition to the controlgrids of the tub s of each of said pairs oi tubes, the phase e othersaid hands applied to the consrol grids ci the tubes ci one of saidpairs of tubes being substantially reversed relative to the phase oi *hesaid other oi said side bands but relatively reversed phase or polarityas applied to the control electrodes ci one of the other of said oftubes, means for applying energy charac tic tubes or" said carrier waveenergy in phase to the control grids of the tubes in each of said pairsof tubes, output circuits conect' ig the anodes oi each of said pairs oftubes in push-pull relation, pair of secondary Windirgs couple to eachof said output circuits and r co circuits connecting a winding in eachor said pairs or windings in series, the polarity of the windings in oneor" said recording circuits being similar so that one component ofoutput currents therein add, the polarity of the windings in the otherol said recording circuits being opposed so that another component ofoutput currents therein also add.

lo. In a system for demodulating Wave energy modulated in phase andcomprising a carrier and two sidebands, means for receiving said energy,iltering means connected with said last named means for separating saidenergy to obtain therefrom carrier energy only and upper and lowersideband energies only, a pair oi electron discharge devices each havingcorresponding control electrodes and corresponding output electrodes,separate means for impressing both of said sideband energies in phaseopposition on the corresponding control electrodes in said devices,means for impressing carrier energy in phase on corresponding controlelectrodes in said devices and an output circuit connected with theoutput electrodes of said devices.

ll. in a system for demodulating phase modulated wave energy comprisinga carrier and upper and lower side'oands and for using the difference infrequency of said sidebands to overcome iading, means for receiving andseparating the carrier and sidebands each from the other, a detector ioreach sideband each detector having a plurality or electrodes includingan output electrode, means for impressing one sideband on electrodes inone oi said detectors and the other sideband on the correspondingelectrodes in the other detector, means for impressing carrier waveenergy displaced in phase relative to both sidehands on both detectorsand a separate output circuit coupled to the output electrodes of eachof said detectors.

l2. in a system for receiving wave energy modulated in phase andcomprising carrier and sideband components, means for separating saidcarrier and sideband components each from the other and passing theseparated components over separate paths, a pair ol' diierentialdetectors, each detector comprising two discharge devices having controlelectrodes and output electrodes, means for connecting one of the pathsin which sideband energy flows to like control electrodes in th'vdevices of one of said pairs of detectors, means connecting the path inwhich the other sideband energy flows to like electrodes in the devicesoi the other or said pair of detectors, means connecting the path inwhich carrier energy flows to corresponding electrodes in each ol` thedevices of both detectors and an indicating circuit coupled tocorresponding electrodes in each of the devices in both of saiddetectors.

i3. A system as recited in claim l2 wherein the carrier Waves applied tothe corresponding electrodes in each of the devices i both detectors aredisplaced in phase relative to the energy in both of said sidebands.

in a phase modulation receiver, a plurality or" pairs of electrondischarge devices each having a control grid, a cathode and an anode, asource of phase modulated oscillatory energy comprising a carrier andsidebands, means for applying one of said sidebands in phase oppositionto 'the control grids of the tubes of each of said pairs ci tuoes, meansfor applying the other or said side'cands reversed in phase relative tothe phase of said first sidehand, in phase opposition to the controelectrodes of the tubes of each or' said pairs oi tubes, means forapplying energy characteristic oi said carrier wave energy in phase tose control gr or the tubes in each or" said pairs or" tubes, thein-phase carrier energy on the control grids of one of said pairs oftubes being in phase or in phase opposition relative to the inphasecarrier energy on the control grids of the other of said pairs of tubes,output circuits connecting the anodes of each of said pairs of tubes inpush-pull relation, a pair of secondary windings coupled to each of saidoutput circuits and recording circuits connecting a winding in each ofsaid pairs of windings in series, the p0- larity of the windings in oneof said circuits being similar so that currents therein add, thepolarity of the windings in the other of said circuits being opposedso'that the currents therein oppose.

15. In a system for demodulating wave energy modulated in phase andcomprising a carrier and two sidebands, means for receiving said energy,means connected with said last named means for separating said energy toobtain therefrom carrier and upper and lower sideband energy, a pair ofelectron discharge devices having corresponding control electrodes andoutput electrodes, separate means for impressing both of said sidebandenergies in phase oppositiony to the corresponding control electrodes insaid devices, means for impressing carrier energy in phase oncorresponding control electrodes in said devices and an indicatingcircuit connected with the output electrodes of said devices.

16. A system for receiving and demodulating modulated transmitted waveenergy comprising two sidebands resulting from modulating a carrier atsignal frequency and preventing phase variations in said carrier duringtransmission from reducing the modulation output comprising, a receiverincluding means responsive to said modulated Wave, two separateiiltering means coupled with said responsive means for derivingrespectively lower sideband energy only and upper sideband energy only,means for producing oscillatory energy of a frequency of the order ofsaid carrier frequency and of substantially constant amplitude, aseparate balanced demodulating means coupled to each of said filteringmeans, means coupling each of said balanced demodulating means to saidoscillation producing means so that oscillatory energy is beat with eachsideband separately to produce two independent beat notes characteristicof each sideband, independent output circuits connected with each ofsaid demodulating means in each of which outputs beat frequency energysubstantially reproducing the original modulation regardless of phase orfrequency iiuctuations of the received carrier ows, and means coupledwith said output circuits for adding said output energies irrespectiveof the phases to render the signals.

17. In a system. for receiving and demodulating modulated transmittedwave energy comprising a carrier and two sidebands and preventingvariations in the phase of said carrier during transmission fromreducing the modulated output, modulated wave responsive means, sidebandseparating means coupled with said responsive means, carrier separatingmeans coupled with said responsive means, a pair of balanced rectifiers,means for impressing one sideband and carrier energy on one of saidbalanced rectiers, means for impressing the other sideband and carrierenergy on the other of said balanced rectiers, independent outputcircuits connected with each of said rectiiiers in each of which outputcircuits energy substantially reproducing the original modulationregardless of phase or frequency uctuations of the received carrierflows, and means associated with said output circuits for recording thearithmetic sumr of said outputs to render the signals.

18. 'Ihe method of demodulating transmitted Wave energy comprising acarrier and sidebands and for Apreventing phase or frequencyfluctuations of the carrier during transmission from reducing thedemcdulated output which includes the steps of beating two portions ofeach of said sideband energies with carrier energy to produce pairs ofbeat notes characteristic of upper and lower sideband energy, adjustingthe phase relation of the carrier and sideband portions so that adisplaced phase relation is maintained between said pairs of beat noteswhereby beat notes characteristic of the same sideband energy do notoppose and cancel, and combining the beat notes characteristic of eachsideband energy additively to reproduce the signal.

19. In a system for demodulating transmitted wave energy comprisingcarrier energy and a pair of sidebands and for preventing phase orfrequency fluctuations of the carrier during transmission from opposingdemodulation components and reducing the output, means for beatingenergy characteristic of said carrier energy with two portions of eachof said sideband energies to produce beat note energies of displacedphase relation, and means for combining the beat note energiescharacteristic of each sideband additively to produce two independentoutputs.

20. 'I'he method of reducing the effects of fading in radiocommunications which includes the steps of, modulating a carrier wave atthe transmitter to produce at least two side frequencies correspondingto each modulating frequency, transmitting the resulting side bandfrequencies, receiving the transmitted side band frequencies, producing,by means of said received side band frequencies, new bands of waveenergy characteristic of the original sideband frequencies, producinglocal oscillations of substantially constant amplitude and of afrequency intermediate the mean frequencies of said produced bands ofwave energy, separately beating said local oscillations with each ofsaid produced bands of wave energy to produce, by each of said beatingprocesses, two components of diiferent phase each characteristic ofsignal modulations and combining pairs of said components characteristicof the same band of Wave energy additively to render the signal.

2l. In a system of radio communication, a transmitter in which theamplitude or strength of a wave is signal modulated to produce at leasttwo side frequencies corresponding to each modulating frequency and theresultant carrier and side band are transmitted, a receiver energized bysaid transmitted side bands, means in said receiver for separating theside bands from the carrier and from each other and for deriving, fromsaid side bands, bands of energy characteristic of each side band, apair of rectifying means, means for impressing a band of energycharacteristic of each side band on each of said rectifying means, meansfor impressing beating voltages on each of said rectifying means, meanscoupled with said rectifying means to produce an independent output foreach side band each output substantially reproducing the originalmodulation irrespective of phase or frequency fluctuations of thereceived carrier, and means for recording the two outputs singly andtogether to render intelligible signals.

CLARENCE W. HAN SELL.

